Production of nitrocellulose



UNITED STATES PATENT OFFICE PRODUCTION OF NITROCELLULOSE Milton 0. Schur, Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporation oi. Maine N0 Drawing. Application Julie 17, 1930, Serial No. 461,854. Renewed July 8, 1935 13 Claims.

This invention relates to the production of nitrocellulose, and more particularly nitrocellulose characterized by low solution viscosity, 1. e., the capacity of being dissolved in suitable solvent media to form solutions of low viscosity.

In accordance with one phase of the present invention, the cellulose fiber used as a raw material is modified while in sheet form by the use of suitable reagents before it is converted into nitrocellulose. Aside from the fact that the pretreatment alters the nature of the cellulose, so that nitrocellulose prepared therefrom has low solution viscosity, the use of the fiber in sheet form makes possible ease and economy of handling, high yields, and other advantageous results which will hereinafter appear in greater particularity.

While not restricted thereto, the principles of the present invention are applicable with especial advantage in the production of nitrocelluloses of low viscosity, such as are used asa base in the manufacture of lacquers and films. In the preparation of nitrocelluloses of this kind, it has been customary to reduce viscosity either by heating the nitrocellulose in water under pressure or by digesting a solution of the nitrocellulose in an inert solvent containing a suitable reagent, such as pyridine.- Both these methods are subject to drawbacks. The first is dangerous and has led to several disastrous explosions; the second tends to give dark-colored solutions and unstable nitrocellulose films. It has been proposed to treat cellulose in bulk condition with acid reagents prior to nitration, to reduce the size of the cellulose molecular aggregate and thereby to lead to a low viscosity nitrocellulose directly upon nitration, but if cellulose is given a preliminary acid treatment in such condition, the mechanical loss attending the shortening of the fiber which accompanies the initial hydrolysis makes the process impracticable, especially if the production of nitrocelluloses of the lowest viscosities, e. g., the so-called /,--second variety, is in view.

I have discovered that if the cellulose is given the preliminary acid treatment while in the form of sheets, including tissue, paper, and pulp boards known as drier sheets, the sheets remain substantially intact and the mechanical losses are consequently negligibly small. Notwithstanding the fact that the fiber units within the sheets may be reduced in size to a greater or less degree according to the nature of the initial treatment, the fibers, being well interfelted, hold together, and can be washed and dried or directly dried without loss, and similarly can be nltrated with substantially no mechanical loss.

The initial acid treatment may be carried out in various ways and upon sheets of various sizes, including small pieces or shreds of tissue or comparatively large, thick sheets of unbeaten pulp. The sheets may be immersed in a solution of the acid, but if the'acid is one which is volatile, such as the hydrochloric and nitric acids presently mentioned, they may be exposed to the action of the acid fumes or vapors. Among the "various acid reagents which may be used for the treatment are the strong mineral acids, such as hydrochloric, sulphuric, and nitric, but they should be used preferably as comparatively dilute solutions or, if feasible, in the form of acid vapors, in order to avoid the severe degradation and loss of cellulose occurring in concentrated solutions. The acid-treated sheets may be washed, but this operation is unnecessary, especially when the sheets are carefully dried to prevent charring. The sheets are preferably dried before they are subjected to the action 01' the nitrating acid, in order to avoid dilution of such acid.

Wood pulp such as is of high alpha cellulose content and otherwise suitable for use in the production of nitrocelluloses offers an important practical advantage when used as the raw material. This advantage resides in the fact that pulp mills are generally equipped to market, and in fact usually do market, unbeaten wood pulp in the form of boards or comparatively thick sheets. These sheets are known as drier sheets f or the reason that they are produced by passing the processed wood pulp as an aqueous suspension over pulp driers, which may be built as large units capable of handling large amounts of pulp, say, 200 tons or more, of dry fiber per day. Such sheets are currently being sold, for example, for manufacture into artificial silk by the viscoserayon process, wherein they are converted as such into alkali cellulose, but they may be used to even greater advantage in nitrocellulose manufacture as hereinbefore described, since they may be maintained in sheet form to the finished product, which, too, may be marketed as sheets. A specific example of procedure falling within the purview of the present invention may be carried out substantially as follows. wood pulp of high alpha cellulose content, say. 0.035 inches thick, are suspended on a rack in 1% HCl solution at C. for six hours. They are then drained free of excess acid and are washed to remove residual acid contained there- Drier sheets of refined i in. The washed sheets are then partially dewatered by squeezing between rolls, whereupon they are dried as in an atmosphere or warm air. The dried sheets are then nitrated, preferably in two or more steps, as-disclosed and claimed in my application Serial No. 451,995, filed May 13, 1930 (now Patent 1,913,416, dated June 13, 1933). For instance, they may be initially immersed for three hours in a nitrating acid consisting of 17.0% H1O, 33.2% HNOa, and 49.8% H2304, at 45 C. This nitrating acid is then drained off and a second nitrating acid, consisting of 17.5% H2O, 20.6% HNOz, and 61.9% H2804, at 40 C., is added and the sheets permitted to remain steeped one hour therein. The second acid is then drained oif and the sheets are drowned in water, washed, stabilized, again washed, and dried. The resulting nitrocellulose is very low in vis-. cosity and the yield satisfactory, the results being vastly superior to those realized when cellulose in bulk form is similarly pretreated with acid and then nitrated.

Another example of procedure in accordance with the present invention may be carried out substantially as follows. A paper made from refined wood pulp. of high alpha cellulose content, of a thickness of 0.0025 inches, is cut into pieces about inch by 1 inch, and the pieces are added to and suspended in a 0.1% KG] so-' lution at C., where they are permitted to remain for sixteen hours. The acid-treated pieces are then drained on a wire screen and are dried, say, at C. The dried pieces are treated with a nitrating acid of suitable composition, being immersed, for instance, for thirty minutes in a nitrating acid consisting of 18.7% H2O, 20.5% HNOs, and 60.8% H2804, at 40 C., the ratio of acid to fiber being 50 to 1. The nitrated pieces are freed from the spent acid in the customary manner, with the aid of a centrifuge. The pieces of nitrocellulose are firm and hold together during the centrifuging operation, so that no abnormally high loss of nitrocellulose through the holes in the centrifuge basket takes place. The centrifuged nitrocellulose is now stabilized, washed, and dehydrated as usual. The viscosity of the product in the different nitrocellulose solvents is very low, being of the order of viscosity of the nitrocelluloses suitable for use in lacquer preparation. The over-all yield of nitrocellulose, figured on the initial bone-dry weight of the paper, is satisfactorily high, approaching a theoretically perfect yield.

Films prepared from the nitrocelluloses made according to the foregoing procedures were normal in strength and in stability.

By the expression sheet form or sheets, as used in the appended claims, unless I further characterize or qualify the sheets, I mean cellu lose fiber in a previously felted condition, irrespective of whether the sheets are of comparatively large area or have been cut into pieces of small size, as distinguished from pulp or cotton linters in bulk form, which when mixed with an aqueous medium result in a suspension of more or less individualized fibers. When small pieces of paper are used, the nitrating operation may be more or less analogous to that customarily employed in the industry. When using sheets of comparatively large area, such as drier sheets, however, the various treating steps may be performed while the sheets are supported in frames or on racks or are suspended from rods or clamps. The sheets are preferably entirely submerged in the treating reagents or water in spaced, parallel relation, so that the reagents or water will contact uniformly with all the surface presented by the sheets and thus ensure uniform treatment.

While I have hereinbefore described specifically the use of acid substance for reducing the solution viscosity of cellulose fiber in sheet form, yet my invention comprehends the use of other than acid reagents capable of reducing the solution viscosity of the fiber when the fiber is in the form of comparatively thick sheets of wood pulp,-the form of raw material which I prefer to employ because of its comparatively low cost and the convenience and economy in processing sheets, as well as the avoidance of mechanical loss in such processing. Such drier sheets are preferably composed of a pure white, highly reflned wood pulp in unbeaten condition, particularly when used for nitrocellulose manufacture. In eifecting a reduction of the solution viscosity of pulp in such form, solutions of oxidizing agents such as potassium permanganate or ammonium persulphate, of mercerizing solutions containing, if desired, a suitable oxidizing agent may be used. A suitable oxidizing agent may also be added to an acid reagent with which the sheets are to be treated. The reagent for reducing the solution viscosity of the fiber may, however, be non-acid, the desired effect being produced, for example, with neutral or alkaline solutions of potassium permanganate. When such sheets are treated with mercerizing solutions in accordance with the present invention, the sheets are preferably washed after such treatment, and, if desired, also treated with dilute acid solution and again washed to remove alkali present therein, whereupon the 1 sheets are dried and nitrated as such. Such practice is distinctly demarcated from that followed in the viscose-rayon industry, wherein the mercerized sheets are pulped up and then treated with carbon bisulphide while still in the form of soda cellulose and without intermediate drying.

In making derivatives of exceedingly low solution viscosities, for instance nitrocelluloses of the /z-S8c0nd variety or lower, it is of advantage to combine two or more pretreatments, as one may realize a cumulative eflfect of such treatments. A combination of pretreatments which has been found especially eifective and advantageous in reducing solution viscosity of cellulose fiber is a mercerization of the fiber, followed by the hydrolysis of the fiber in sheet form while maintaining the sheets intact. The mercerization of the fiber, as well as its hydrolysis, is preferably effected with the fiber in the form of drier sheets, not only for the reasons hereinbefore noted, but further because it is much more diflicult to make uniform sheets of mercerized fiber than of unmercerized fiber, owing to the curly nature of the former, its exceedingly high freeness, and its pronounced tendency to gather into clumps. Further, when pulp is mercerized in sheet form, excess alkali may be readily removed and recovered therefrom by pressing, and residual alkali is easily washed therefrom, whereas it is a difiicult matter to remove alkali from cellulose fiber mercerized in bulk, as in such latter case the mass is highly gelatinous and does not readily release entrapped alkali when subjected to draining, squeezing, or washing operations. Also, sheets of unmercerized pulp are much stronger and more tenacious than sheets of pulp mercerized in bulk. Again, the mercerization of the drier sheet toughens it so that its integrity or capacity for holding together during the hydrolytic treatment is greatly enhanced, particularly if the mercerized sheet is steeped in hot water as opposed to cold water after the mercerizing solution has been drained oil. A two-steppretreatment along this line, using drier sheets of the type hereinbefore described as a raw material, may be practiced with very gratifying results, substantially as follows. The drier sheets are initially steeped for A to 4 hours in an 18% caustic soda solution at 18 to 20 0., and the solution-soaked sheets are then pressed free of excess solution. Both these operations may be performed in a steepingpress, such as is used in the viscose industry. The

.pressed, mercerized sheets. containing residual mercerizing solution are in afirm, intact condition. They may hence be plunged into a bath of preferably hot wash water in which they hold together entirely satisfactorily. Preferably after several changes of hot water in the bath, the water is drained oil and a 1% solution of hydrochloric acid at 90 to 95 C. is added to'the sheets, in which the sheets are "allowed to remain submersed for about V to 2 hours. The acid solution is then drained of! and the sheets are again washed, preferably with hot water and with several changes of such water. The water-soaked sheets are then squeezed free of excess water, and the moist sheets are dried as on a rack in an atmosphere-of warm air. Whenthe dried sheets were nitrated in two stages, as hereinbefore described, substantially theoretical yields of nitrocellulose having a viscosity solution of 1/20 second, and even lower, were produced. Nitrocellulose solutions prepared from the nitrated product were of excellent water-white color and clarity.

In certain instances, it may be expedient to mercerize the pulp in bulk form and then to hydrolyze the mercerized pulp after it has been made up into sheets, as in such instance, too, the treatment of the mercerized fiber in sheet form makes possible the advantage hereinbeforenoted duringthe hydrolytic treatment and the nitrating operation. To reiterate, amongthese advantages may be mentioned simplicity of apparatus required, the avoidance of mechanical loss, and the desirability of drier sheets as the starting material for conversion into nitrocelluloses.

In accordance with another phase of my invention, I use mercerizing solutions at above room temperature in the treatment of cellulose fiber, as I have discovered that solution viscosity reduction of the fiber is greatly favored by high temperature, as well as by mercerization. I thus take advantage of the eflect of mercerlzation and.

oi. heat in breaking down the size of the cellulose molecular aggregate and hence secure better results than could be had through either effect alone. This discovery may be applied independently of the hydrolytic treatment upon bulk fiber in the form of drier sheets or waterleaf papers. The fiber may be cotton linters, as well as wood pulp or cellulose fiber of other derivations. Ordinarily, mercerizations are carried out at room temperature or lower, since as the temperature of the caustic soda solution is raised,-the concentration of caustic soda in solution must be increased in order to eifect mercerization; but in view-oi my discovery that a solution of caustic soda which mercerizes at above room temperahours in a Wt 2% solution of caustic soda at 125 C. under atmospheric pressure conditions, as such a strong solution mercerizes at this high tempera-- ture. When sheets so mercerized are drained, washed with .hot water, dried,and nitrated in two stages at 40 C., as hereinbefore described, an excellent yield of nitrocellulose of -second solution viscosity is obtained. When sheets so mercerized were washed, then steeped in .a 1% hydrochloric acid solution for one hour at 90 0.,

then again washed, dried, and nitrated in two stagesat 30C. as hereinbefore described, an excellent yield of nitrocellulose of l/25-second solution viscosity was realized.

The mercerizing solution may be used at a temperature above the normal boiling point by enclosure in a pressure vessel or digester, but in such case very highly concentrated caustic soda solutions are necessary to bring about mercerization. In order to avoid the necessity of using highly concentrated caustic soda solutions for mercerizing, a solution of, say, 18% caustic soda may be used at room temperature or lower to efiect mercerization of the fiber, and after the fiber has been thoroughly saturated with the solution and inercer'ized, the solution may be heated considerably to produce a change of solution viscosity incident to a temperature which is far above the mercerizing temperature of the solution. As already noted, the mercerizing solution may contain oxidizing agents such as sodium permanganate, sodium hypochlorite, or ammonium persulphate, as these promote a reduction of the solution viscosity of the fiber. The mercerizing solution may also contain various reducing agents, such as sodium sulphide, or sodium sulphite. Before washing the mercerized sheets, they may be aged as 'in the viscose-rayon industry, as ageing also brings about a lowering of the solution viscosity of the fiber. When a subsequent hydrolyzing' treatment of the fiber is effected, various hydrolyzing media may be used, including acid or acid salt solutions, hot water under pressure,

. hot dilute solutions of mildly alkaline substance claim broadly the mercerization of cellulose fiber,

followed by a hydrolytic treatment of the mer-.

cerized fiber, as this is disclosed and claimed in application Ser. No. 430,147, filed February 20,

1930, by George A. Richter.

By the two-step pretreatment of cellulose fiber as hereinbefore described prior to its nitration, it is possible to produce nitrocelluloses of a solution viscosity below A-second, which are capable of forming clear solutions substantially free of coloring matter and capable of forming films. So far as I know, no one has heretofore produced nitrocelluloses of this nature, and more particularly in the form of sheets, for the processes heretofore used do not lend themselves to the production of nitrocellulose lower than A -second in solution viscosity and having the characteristics hereinbefore noted.

I claim:

l. A process which comprises mercerizing cellulose fiber, hydrolyzing the fiber in interfelted, sheet form, and nitrating the hydrolyzed sheet.

2 A process which comprises mercerizing comparatively thick sheets of interfelted, wood pulp fibers, hydrolyzing the mercerized sheets, and then nitrating the sheets.

3. A process which comprises treating cellulose fiber at not materially above about room temperature with a caustic soda solution capable of effecting mercerization at about room temperature but not materially above such temperature, and after the fiber has been mereerised heating the solution to a temperature materially above room temperature.

4. A process which comprises treating thick, porous sheets of interielted, wood pulp fibers with a solution of mineral acid a strength not exceedlng about 1%, and nitrating the treated sheets.

5. A process which comprises treating thick, porous sheets of lntertelted, wood pulp fibers with a solution of hydrochloric acid of about 0.1% to 1% strength, and nitrating the treated sheets.

6. A process which comprises treating thick, porous sheets of interielted, wood pulp fibers with a solution of hydrochloric acid of about 0.1% to 1% strength, washing the treated sheets, drying them, and nitrating the dried sheets.

7. A process which comprises meroerising thick, porous sheets of interfelted, wood pulp fibers, treating the mercerized sheets with a dilute solution of mineral acid, and nitrating the sheets.

8. A process which comprises mercerizlng cellulose fibers, treating the merceriaed fibers in interfelted sheet form with a dilute solution of mineral acid to reduce the solution viscosity of the fibers while preserving their interielted cohesion and thereby malntainingfine fibers and fiber fragments within the sheets, and nitrating such treated sheets.

9. A process which comprises mercerizing cellulose fibers, steeping the mercerized fibers in interi'elted sheet form in a dilute solution oi. mineral acid to reduce the solution viscodty of the fibers while preserving their interielted cohesion and thereby maintaining the fine fibers and fiber fragments within the sheets, removing the sheets from the steeping solution, and nitrating them.

10. The process which comprises hydrolyzing sheets of interfelted cellulose fibres derived from wood pulp with a dilute solution 0! a mineral acid, thereby reducing the size of fibre units in the sheets while maintaining the intertelted structure of the sheets and without substantial loss 0! such reduced fibre units, and nitratlng such acid hydrolyzed sheets while preserving their intertelted structure and without substantial loss of said nitrated reduced fibre units, thereby forming sheets having a low solution viscosity.

11. A process which comprises hydrolyzing sheets of interielted cellulose fibers with a dilute solution oi! mineral acid, thereby reducing the solution viscosity of the cellulose and reducing the size of fiber units in the sheets but without substantial loss of such reduced fiber units, and nitrating the resulting acid-hydrolyzed cellulose fibers associated with substantially all of such reduced fiber units to produce nitrocellulose or low solution viscosity.

12. A process which comprises hydrolyzing sheets of intertelted wood pulp fibers with a dilute solution of mineral acid, thereby reducing the solution viscosity of the cellulose and reducing the size'oi fiber units in the sheets but without substantial loss of such reduced fiber units, and nitrating the resulting acid-hydrolyzed wood pulp fibers associated with substantially all or such reduced fiber units to produce nitrocellulose of low solution viscosity.

13. A process which comprises hydrolyzing sheets of interfeltedv cellulose fibers with a dilute solution of mineral acid, thereby reducing the size of fiber units in the sheets while maintaining the interielted structure of the sheets and without substantial loss oi such reduced fiber units, and nitrating such acid-hydrolyzed sheets while preserving their interielted structure and without substantial loss of said nitrated reduced fiber units, thereby forming sheets having a low solution viscosity.

' MILTON O. SCHUR. 

